Traditional diagnostic radiography uses x-ray generators that emit X-rays over a broad energy band. A large fraction of this band contains x-rays which are not useful for medical imaging because their energy is either too high to interact in the tissue being examined or too low to reach the X-ray detector or film used to record them. The x-rays with too low an energy to reach the detector are especially problematic because they unnecessarily expose normal tissue and raise the radiation dose received by the patient. It has long been realized that the use of monochromatic x-rays, if available at the appropriate energy, would provide optimal diagnostic images while minimizing the radiation dose. To date, no such monochromatic X-ray source has been available for routine clinical diagnostic use.
Monochromatic radiation has been used in specialized settings. However, conventional systems for generating monochromatic radiation have been unsuitable for clinical or routine commercial use due to their prohibitive size, cost and/or complexity. For example, monochromatic X-rays can be copiously produced in synchrotron sources utilizing an inefficient Bragg crystal as a filter or using a solid, flat target x-ray fluorescer but these are very large and not practical for routine use in hospitals and clinics.
Monochromatic x-rays may be generated by providing in series a target (also referred to as the anode) that produces broad spectrum radiation in response to an incident electron beam, followed by a fluorescing target that produces monochromatic x-rays in response to incident broad spectrum radiation. The term “broad spectrum radiation” is used herein to describe Bremsstrahlung radiation with or without characteristic emission lines of the anode material. Briefly, the principles of producing monochromatic x-rays via x-ray fluorescence are as follows.
Thick Target Bremsstrahlung
In an x-ray tube electrons are liberated from a heated filament called the cathode and accelerated by a high voltage (e.g., ˜50 kV) toward a metal target called the anode as illustrated schematically in FIG. 1. The high energy electrons interact with the atoms in the anode. Often an electron with energy E1 comes close to a nucleus in the target and its trajectory is altered by the electromagnetic interaction. In this deflection process, it decelerates toward the nucleus. As it slows to an energy E2, it emits an X-ray photon with energy E2-E1. This radiation is called Bremsstrahlung radiation (braking radiation) and the kinematics are shown in FIG. 2.
The energy of the emitted photon can take any value up to the maximum energy of the incident electron, Emax. As the electron is not destroyed it can undergo multiple interactions until it loses all of its energy or combines with an atom in the anode. Initial interactions will vary from minor to major energy changes depending on the actual angle and proximity to the nucleus. As a result, Bremsstrahlung radiation will have a generally continuous spectrum, as shown in FIG. 3. The probability of Bremsstrahlung production is proportional to Z2, where Z is the atomic number of the target material, and the efficiency of production is proportional to Z and the x-ray tube voltage. Note that low energy Bremsstrahlung X-rays are absorbed by the thick target anode as they try to escape from deep inside causing the intensity curve to bend over at the lowest energies, as discussed in further detail below.
Characteristic Line Emission
While most of the electrons slow down and have their trajectories changed, some will collide with electrons that are bound by an energy, BE, in their respective orbitals or shells that surround the nucleus in the target atom. As shown in FIG. 4, these shells are denoted by K, L, M, N, etc. In the collision between the incoming electron and the bound electron, the bound electron will be ejected from the atom if the energy of the incoming electron is greater than BE of the orbiting electron. For example, the impacting electron with energy E>BEK, shown in FIG. 4, will eject the K-shell electron leaving a vacancy in the K shell. The resulting excited and ionized atom will de-excite as an electron in an outer orbit will fill the vacancy. During the de-excitation, an X-ray is emitted with an energy equal to the difference between the initial and final energy levels of the electron involved with the de-excitation. Since the energy levels of the orbital shells are unique to each element on the Periodic Chart, the energy of the X-ray identifies the element. The energy will be monoenergetic and the spectrum appears monochromatic rather than a broad continuous band. Here, monochromatic means that the width in energy of the emission line is equal to the natural line width associated with the atomic transition involved. For copper Kα x-rays, the natural line width is about 4 eV. For Zr Kα, Mo Kα and Pt Kα, the line widths are approximately, 5.7 eV, 6.8 eV and 60 eV, respectively. The complete spectrum from an X-ray tube with a molybdenum target as the anode is shown in FIG. 5. The characteristic emission lines unique to the atomic energy levels of molybdenum are shown superimposed on the thick target Bremsstrahlung.
X-Ray Absorption and X-Ray Fluorescence
When an x-ray from any type of x-ray source strikes a sample, the x-ray can either be absorbed by an atom or scattered through the material. The process in which an x-ray is absorbed by an atom by transferring all of its energy to an innermost electron is called the photoelectric effect, as illustrated in FIG. 6A. This occurs when the incident x-ray has more energy than the binding energy of the orbital electron it encounters in a collision. In the interaction the photon ceases to exist imparting all of its energy to the orbital electron. Most of the x-ray energy is required to overcome the binding energy of the orbital electron and the remainder is imparted to the electron upon its ejection leaving a vacancy in the shell. The ejected free electron is called a photoelectron. A photoelectric interaction is most likely to occur when the energy of the incident photon exceeds but is relatively close to the binding energy of the electron it strikes.
As an example, a photoelectric interaction is more likely to occur for a K-shell electron with a binding energy of 23.2 keV when the incident photon is 25 keV than if it were 50 keV. This is because the photoelectric effect is inversely proportional to approximately the third power of the X-ray energy. This fall-off is interrupted by a sharp rise when the x-ray energy is equal to the binding energy of an electron shell (K, L, M, etc.) in the absorber. The lowest energy at which a vacancy can be created in the particular shell and is referred to as the edge. FIG. 7 shows the absorption of tin (Sn) as a function of x-ray energy. The absorption is defined on the ordinate axis by its mass attenuation coefficient. The absorption edges corresponding to the binding energies of the L orbitals and the K orbitals are shown by the discontinuous jumps at approximately 43.4 keV and 29 keV, respectively. Every element on the Periodic Chart has a similar curve describing its absorption as a function of x-ray energy.
The vacancies in the inner shell of the atom present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process emit a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells as described above in the section on Characteristic Line Emission. This photon-induced process of x-ray emission is called X-ray Fluorescence, or XRF. FIG. 6B shows schematically X-ray fluorescence from the K shell and a typical x-ray fluorescence spectrum from a sample of aluminum is shown in FIG. 8. The spectrum is measured with a solid state, photon counting detector whose energy resolution dominates the natural line width of the L-K transition. It is important to note that these monoenergetic emission lines do not sit on top of a background of broad band continuous radiation; rather, the spectrum is Bremsstrahlung free.